1. Field of the Invention
The invention relates to telecommunications. In particular, the invention relates to a novel and improved adaptive link adaptation method and system for a packet data enabled mobile telecommunication network.
2. Description of the Related Art
Recently mobile telecommunication networks have started to provide packet data services for subscribers in addition to traditional circuit switched services. A circuit switched service refers to a type of service for which a physical path is dedicated to a single connection between two end-points in the network for the duration of the connection. For example ordinary voice phone service is circuit-switched. Packet switched data service refers to a type of service in which relatively small units of data called packets are routed through a network based on the destination address contained within each packet. In the following the terms packet switched data and packet data are used interchangeably unless otherwise noted.
An example of packet data service for mobile telecommunication networks is General Packet Radio Service (GPRS). GPRS is designed to support especially digital mobile telecommunication networks based on the GSM (Global System for Mobile Communications) standard. However, GPRS is not restricted to only GSM networks but may support for example 3GPP (Third Generation Partnership Project) system based digital mobile telecommunication networks. A recent enhanced version of GPRS is referred to as EGPRS (EDGE General Packet Radio Service, where EDGE stands for Enhanced Data rates for GSM Evolution).
A GPRS enabled mobile telecommunication network includes two additional network elements or nodes. These are a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node (GGSN). Additionally GPRS requires some modifications to existing network elements. A Packet Control Unit (PCU) is typically implemented in connection with a base station or base station controller. The Packet Control Unit controls functionalities associated with packet switched data transfer. Further, mobile terminals need to support packet switched data transfer.
Radio links connecting a PCU to mobile terminals are one of the most critical parts of GPRS and EGPRS enabled mobile telecommunication networks. For example, radio signal disturbance due to fast-fading, shadowing, noise and co-channel interference may result in data packets being lost and thus needing to be re-transmitted. Thus, various coding schemes are used to avoid transmission errors.
When a data packet is coded, some redundant information is added to the source data. When the same packet is later decoded, this redundant information is used to correct and/or detect bit errors that occurred during transmission.
Error correction can be improved by increasing the amount of redundant information in a data packet but at the same time net bit rate is reduced. Hence it would be advantageous to use a robust coding scheme for poor radio conditions, and a coding scheme with high net bit rate when radio conditions are good. GPRS provides four different coding schemes, whereas EGPRS provides nine different coding schemes.
Various link adaptation (LA) algorithms are used to select an optimum coding scheme for a particular radio link. In a typical link adaptation method a receiver, typically a mobile terminal, measures channel quality of the used radio link in terms of signal strength, bit error rate, packet error rate or some other measure appropriate for this purpose. A channel quality estimate is then fed into the link adaptation algorithm which makes the decision to change or not to change the current coding scheme by comparing the estimated channel quality with certain threshold values.
Prior art link adaptation techniques are disclosed, for example, in U.S. Pat. No. 5,359,607; U.S. Pat. No. 5,701,294; U.S. Pat. No. 5,862,171; U.S. Pat. No. 6,072,990; U.S. Pat. No. 6,134,220; U.S. Pat. No. 6,385,462; U.S. Pat. No. 6,308,082; U.S. Pat. No. 6,122,293 and WO 023700.
However, there are significant drawbacks with the prior art link adaptation techniques. They all require pre-determined threshold values to compare the estimated channel quality with. Typically these pre-determined threshold values are obtained by measuring the performance of each coding scheme beforehand at various radio conditions. This can be done either by computer simulations or by empirical measurements. In this context computer simulations and empirical measurements are far from idea solutions, however.
Computer simulations contain assumptions e.g. about the radio channel type. By their very nature these assumptions cannot cover all real world situations, thus errors result. For example, in the context of computer simulations TU3idealFH refers to a commonly used type of channel simulation. The symbol “TU” in TU3idealFH refers to Typical Urban. In real world situations radio channel characteristics differ in a city center, in suburban area and in countryside. Yet the same TU3idealFH channel simulation is typically used for all these cases, thus resulting in inaccuracies. Further, the number “3” in TU3idealFH refers to a mobile terminal assumed to move with the speed of 3 km/h in average. Yet, if the base station is located e.g. along a motorway, the assumption again results in inaccuracies. Further, the term “idealFH” in TU3idealFH refers to idealistic frequency hopping, idealistic meaning that there is one interfering mobile terminal on all the timeslots that belong to the hopping sequence. Yet, if traffic load in the network is low, this assumption again results in inaccuracies.
Empirical measurements on the other hand generate results that are specific to the time and place in which the measurements took place. Further, in practice it is difficult to measure the performance of different coding schemes under all the possible various radio conditions.
Thus there is an obvious need for link adaptation which does not require pre-determined threshold values and which can dynamically adapt to current radio conditions.